Philippe Knauth

Three‐Dimensional Self‐Supported Metal Oxides for Advanced Energy Storage

The miniaturization of power sources aimed at integration into micro- and nano-electronic devices is a big challenge. To ensure the future development of fully autonomous on-board systems, electrodes based on self-supported 3D nanostructured metal oxides have become increasingly important, and their impact is particularly significant when considering the miniaturization of energy storage systems. This review describes recent advances in the development of self-supported 3D nanostructured metal oxides as electrodes for innovative power sources, particularly Li-ion batteries and electrochemical supercapacitors. Current strategies for the design and morphology control of self-supported electrodes fabricated using template, lithography, anodization and self-organized solution techniques are outlined along with different efforts to improve the storage capacity, rate capability, and cyclability.

Electrical and defect thermodynamic properties of nanocrystalline titanium dioxide

The electrical properties of dense nanocrystalline TiO2 ceramics with an average grain size of 35 nm were investigated by impedance spectroscopy and compared with a coarsened material with micron size grains. The nanocrystalline ceramics show an uncommon domain of ionic conductivity at high oxygen pressures and a steep increase of electronic conductivity at low oxygen pressures. These results are discussed in terms of defect chemical models. The enthalpy of reduction of the nanocrystalline TiO2 was deduced to be significantly lower than that of the coarsened material. This may be related to lower defect formation energies at interface sites.

Ionic conductor composites: Theory and materials

The main theoretical concepts on ionic conduction at interfaces, especially the space charge layer model, are summarized in the first part of this review: ion trapping or redistribution leads to charge carrier accumulation, depletion or inversion and, consequently, to conductivity changes in composite materials. Experimental confirmations of the space charge layer model and the complementary percolation model are discussed. Major developments of ionic conductor composite materials over the last 25 years are presented in the second part, including lithium and other alkaline ion conductors, copper and silver ion conductors, di- and trivalent cation and anion conductors, glass and polymer composites. Some future trends and research needs are indicated in conclusion.

Analysis of Temperature-Promoted and Solvent-Assisted Cross-Linking in Sulfonated Poly(ether ether ketone) (SPEEK) Proton-Conducting Membranes

Sulfonated poly(ether ether ketone) (SPEEK) membranes were thermally treated at temperatures between 120 and 160 degrees C. Water uptake measured at different relative humidity values or by full immersion in water between 25 and 145 degrees C was found to depend very strongly on previous thermal treatment and casting solvent. Water-uptake coefficient values as low as 10-15 even upon immersion in water at 100 degrees C were obtained with membranes treated at 160 degrees C. This effect is related to cross-linking by SO2 bridges between macromolecular chains. An important role is also played by the casting solvent: among the investigated solvents, dimethylsulfoxide (DMSO) gave the best results. A chemical kinetics model is outlined that permits the estimation of the relevant kinetic parameters, especially the activation energy of the cross-linking reaction, which was found to be about 60 kJ/mol. These results are of significant importance for the improvement of proton-exchange membrane fuel cells.

Nanostructured negative electrodes based on titania for Li-ion microbatteries

This work reviews recent developments on Li-ion microbatteries. After a short literature overview, use of TiO2 as an alternative anode for Li-ion batteries and enhanced electrochemical performances of nanostructured titania electrodes is introduced. Principle and formation mechanism of self-organized TiO2 nanotubes by electrochemical anodization and electrochemical fabrication of metallic nanowires are discussed in detail. Electrochemical performance of negative electrodes for Li-ion microbatteries composed of self-organized TiO2 nanotubes and composite TiO2 nanotubes–oxide nanowires is presented.

A novel architectured negative electrode based on titania nanotube and iron oxide nanowire composites for Li-ion microbatteries

We report a novel procedure for the fabrication of vertical iron oxide nanowires with quite regular form and diameters ranging between 20 and 150 nm grown from a matrix of self-organized TiO2 nanotubes. The 3 μm thick nanocomposite electrode presented here shows relatively high areal capacities of 468 μAh cm−2 (1st reversible discharge) and 200 μA h cm−2 over 45 cycles at a rate of 25 μA cm−2 (specific capacities of 1190 and 510 mAh g−1, respectively). Additionally, studies performed at kinetics of 6, 12.5, and 50 μA cm−2 suggest that this architectured nanocomposite material reveals excellent electrochemical performance with promising potential applications as nano-architectured negative electrodes for Li-ion microbatteries.

Durability of sulfonated aromatic polymers for proton-exchange-membrane fuel cells.

As a key component of proton-exchange-membrane fuel cells (PEMFCs), proton-exchange membranes (PEMs) must continuously withstand very harsh environments during long-term fuel cell operations. With the coming commercialization of PEMFCs, investigations into the durability and degradation of PEMs are becoming more and more urgent and interesting. Herein, various recent attempts and achievements to improve the durability of sulfonated aromatic polymers (SAPs) are reviewed and some further developments are predicted. Extensive investigations into inexpensive SAPs as alternative electrolyte membranes include modification of available polymer materials; design, synthesis, and optimization of new macromolecules; durability testing; and exploring the degradation mechanisms.

Solute segregation, electrical properties and defect thermodynamics of nanocrystalline TiO2 and CeO2

Abstract The thermodynamic and kinetic properties of nanocrystalline oxides, including nominally undoped TiO 2 (anatase) and Pr- and Cu-doped CeO 2 , are reviewed. The electrical properties of nominally undoped nanocrystalline TiO 2 and CeO 2 differ from conventional microcrystalline materials due to a greatly reduced specific grain boundary impedance and enthalpy of reduction. In TiO 2 , an uncommon domain of ionic conductivity is observed at high oxygen partial pressures, whereas at low P (O 2 ), the electronic conductivity increases strongly with a P (O 2 ) −1/2 dependence. Nanocrystalline CeO 2 , on the other hand, exhibits strongly enhanced oxygen nonstoichiometry and electronic conductivity over the whole P (O 2 ) range. Reduced defect formation energies at interface sites are proposed to be responsible for these properties. The apparent solubility of copper in nanocrystalline CeO 2 –Cu 2 O of about 10 mol% is much enhanced over that of coarse-grained ceria and is accommodated by segregation of copper to the grain boundaries. Nanocrystalline CeO 2 –PrO x , with up to 70 mol% PrO x , is found to be single phase. The oxygen deficiency in this system attains large values ( x >0.1) with evidence for vacancy ordering. The chemical diffusivities (≈10 −6 cm 2 /s) and the low activation energy (≈0.3 eV) suggest short circuiting diffusion paths via interfaces.

Nanocomposite Electrode for Li-Ion Microbatteries Based on SnO on Nanotubular Titania Matrix

A nanocomposite electrode made by electrochemical deposition of Sn on TiO 2 nanotube (ntTiO 2 ) layers and subsequent thermal oxidization to SnO is proposed. X-ray diffraction patterns confirmed the presence of SnO. The spongelike structure of SnO combined with the presence of titania nanotubes is beneficial to buffer large volume changes during reaction with lithium. Galvanostatic discharge/charge tests have been carried out to characterize the electrochemical properties. The electrochemical performance shows that nanocomposite SnO-ntTiO 2 is a promising alternative negative electrode for Li-ion microbatteries.

Electrochemical fabrication of Sn nanowires on titania nanotube guide layers

We describe a novel approach for the fabrication of tailored nanowires using a two-step electrochemical process. It is demonstrated that self-organized TiO(2) nanotubes can be used to activate and guide the electrochemical growth of Sn crystallites, leading to the formation of vertical features with a high aspect ratio. We show that the dimensions and the density of Sn crystallites depend on the electrodeposition parameters.